Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen. Stents have been developed with coatings to deliver drugs or other therapeutic agents. Stents are typically installed with a reduced diameter and deployed to a final diameter. The stents can be self-expanding or can be expanded mechanically.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow.
To prevent restenosis, short flexible cylinders, or stents, constructed of metal or various polymers are implanted within the vessel to maintain lumen size. The stents acts as a scaffold to support the lumen in an open position. Various configurations of stents include a cylindrical tube defined by a mesh, interconnected stents or like segments. Some exemplary stents are disclosed in U.S. Pat. No. 5,292,331 to Boneau, U.S. Pat. No. 6,090,127 to Globerman, U.S. Pat. No. 5,133,732 to Wiktor, U.S. Pat. No. 4,739,762 to Palmaz and U.S. Pat. No. 5,421,955 to Lau. Balloon-expandable stents are mounted on a collapsed balloon at a diameter smaller than when the stents are deployed.
During the procedure, the balloon stent catheter is advanced through a network of tortuous blood vessels. Furthermore, the balloon stent catheter also may encounter narrowed lumens or lumens that are obstructed. Once at the desired site, the balloon is inflated and expands the stent to a final diameter. After deployment, the stent remains in the vessel and the balloon catheter is removed.
The position of the stent on the balloon should be maintained while the balloon stent catheter is moved longitudinally through the network of vessels. In moving to the implant site, the stent may be shifted on the balloon so that the stent may not expand fully along its length or may be completely dislodged from the balloon. Current strategies for retaining the stent on the balloon include plastically deforming the stent so that it is crimped onto the balloon; increasing the friction forces between the stent and balloon by modifying the balloon through heat, pressure, or chemical or adhesive means; adding retainers that physically prevent the stent movement; and combinations thereof.
Stents have been developed with coatings to deliver drug or other therapeutic agents at the site of the stent. Typically, a coating of a soft polymer carrying the drug or therapeutic agent is applied to or bonded with the metal or other material forming the stent. Crimping the stent onto the balloon can damage the soft coating, either causing the coating at the crimp to be thinned or to be lost altogether. Any damage creates uncertainty about the dosage of drug delivered to the patient. Cost may increase because additional drug must be loaded to assure an effective dose. Crimping may also cause the coating material to adhere to the balloon, so that the coating material is withdrawn from the body when the balloon is withdrawn. Crimping force may have to be limited to protect the coating, which can limit the effectiveness of the crimp in holding the stent on the balloon.
WIPO International Publication No. WO 00/45744 to Yang et al. discloses a medical device, such as a stent, which includes a first coating including a drug or therapeutic substance and a relatively inelastic second coating impervious to the therapeutic substance, the second coating fracturing during expansion of the medical device to allow elution of the therapeutic substance through fissures formed through the second coating.
It would be desirable to have an expandable stent with a crimpable coating that would overcome the above disadvantages.